The present invention relates to a powder metallurgic process, and more particularly to a process for manufacturing alloy powder with dual consumable rotary electrodes arc melting.
Since powder metallurgy provides a lot of advantages, it has been widely employed in making not only various products for use in daily life, but also highly advanced scientific and military items. For example, Titanium (Ti) FeAl and TiAl, which is an intermetallic compound, have good high-temperature strength and low density and are therefore very suitable for applications in high-temperature parts for aerospace industry. Currently, there are many ways for manufacturing powder, such as, for example, Gas atomization, water atomization process, chemical reduction process, centrifugal process, mechanical process, electrolytic process, and chemical decomposition process. Among these processes, Gas atomization, water atomization process, mechanical process, and centrifugal process are more suitable for manufacturing alloys powder.
In the Gas atomization process, an amount of gas is supplied into a liquid metal to stir and splash the latter. Refractory material contained in crucible, lead-in tube, or nozzle used in the process tends to dissolve in the liquid metal to contaminate resultant powder. In the water atomization process, an amount of high-pressure water is applied to a liquid metal to atomize the latter. A main drawback of the water atomization process is the water contains high amount of oxygen that tends to react with the metal and produces oxides, besides the content of hydrogen of produced powder is high. Another drawback of the water spray process is it forms powder having relatively irregular shapes and large surface areas, and products sintered from such powder have less compact structure. In addition, the water spray process also has the same problem of contamination of liquid metal and resultant powder by refractory material of crucible, lead-in tube, or nozzle dissolved in the liquid metal. In the general mechanical process, since the produced powder tends to be contaminated by materials forming the liner of machine and the grinding balls, precision contamination-control equipment is required, which undesirably increases the manufacturing cost of the process. For this reason, the mechanical process and rotary electrode arc melting process are normally employed only to manufacture expensive special powder.
A conventional rotary electrode arc melting process has much lower batch productivity as compared to the Gas atomization process and the water atomization process. However, the rotary electrode arc melting process is not subjected to contamination and produces powder within narrow range of powder size. Therefore, this process is highly useful in producing pure clean and round-shaped metal or alloys powder having low surface area to meet the requirement of special powder needed by aerospace industry. In manufacturing alloy powder with the conventional rotary electrode process, it is necessary to pre-melt the alloy and form alloy ingot through pouring practice. The alloy ingot is then formed and machined to produce an electrode. Moreover, tungsten is used as a non-consumable cathode to manufacture powder in the conventional rotary electrode process, and additional works are needed to pre-melt tungsten alloy and form ingots through pouring. It is particularly difficult to produce small-size electrode bars through pouring and the resultant electrode bars tend to have pouring defects. Moreover, the machining of electrode bars made of high hardness and high strength alloy materials tends to caused damaged machining tools. Therefore, omission of the pre-melting step from the conventional rotary electrode process would be helpful in reducing the manufacturing cost and time needed to obtain the alloy powder.
It is therefore a primary object of the present invention to overcome the drawbacks existed in the conventional powder metallurgical processes by providing an improved process for manufacturing powder with rotary electrode and electric arc, in which the step of pre-melting electrode bar as included in the conventional rotary electrode process is omitted.
In the process of the present invention, rotary electrode and tungsten electrode adopted by a conventional rotary electrode and arc process for manufacturing powder are respectively replaced with a rotary or anodic electrode containing a first metal and a feed or cathodic electrode containing a second metal. An inert gas, such as argon, helium, etc., is supplied into equipment for implementing the process to serve as a protective atmosphere and to stabilize a generated electric arc. The equipment has a working voltage within the range from 10 to 90 volts, and preferably within the range from 40 to 70 volts, and a working current within the range from 100 to 1500 A, preferably within the range from 300 to 800 A. Through control of different parameters, including the working current, the working voltage, a distance between the two electrodes, etc., melting rates of the two electrodes may be changed. After the electric arc is generated, the cathodic electrode melts under the high temperature of the arc at a cathodic spot, and droplets of the molten cathodic or second metal are under effects of push force formed by plasma arc flow, electromagnetic acting force, arc force, etc., to spray toward the anodic electrode and mix with molten anodic or first metal. A molten mixture of the first and the second metal is thrown out by a centrifugal force produced by the rotary electrode to form round-shaped alloy powder containing the first and the second metal. According to the process of the present invention, the structure of the alloy powder to be obtained may be decided through change of the melting rates of the two electrodes.